This invention relates generally to ink density control for printing machines, and more specifically relates to an apparatus having an optical densitometer for determining and evaluating ink measuring strips printed on test sheets.
In the state of the art of printing machines, the densities of ink or color balance in printing machines are established by numerous ink-dosing elements arranged across the width of the printing machine and the ink-dosing elements are individually adjustable by remote control adjusting devices. In order to generate the control signals for the automatic ink-dosing elements, ink measuring strips are printed on production sheets and some of these production sheets are selected as test sheets. After printing, the test sheets are conveyed to a densitometer table having an optical densitometer mounted for traversing or scanning the ink measuring strips. In state of the art systems, the signals from the optical densitometer are automatically processed to compare the measured ink densities to desired ink densities in order to remotely control the ink-dosing elements in the printing machine. See, for example, Schramm et al. U.S. Pat. No. 4,200,932 issued Apr. 29, 1980 for which a reexamination certificate issued Apr. 26, 1983. The major printing machine manufacturers sell systems similar to the system disclosed in Schramm et al., U.S. Pat. No. 4,200,932.
Although the ink feed controls for the printing machine are generally automatic, the selection of test sheets and the scanning of the ink measuring strips on the test sheets is a repetitious process. A laborious and sometimes error generating step in the process has been the proper alignment of the test sheets on the densitometer table.
The alignment problem is particularly serious in view of the various types of paper ranging from thin printing paper to thick cards. Card orders require the maximum utilization of the area available for printing and thus for card orders the positions and total area available for ink measuring strips are highly restricted. In practice, the ink measuring strips are typically at different places for different printing orders or jobs depending upon the orientation of the printed text on the sheets.
In order to simplify the recurrent evaluation of the measurement strips on the test sheets by the densitometer, the densitometer is typically mounted on a X-Y position mechanism similar to that used in well known digitally-driven X-Y flat-bed plotters. The plotter is interfaced to a numerical computer by known methods so that the numerical computer will drive the densitometer to any desired coordinates as specified by a procedure executed by the numerical computer. The numerical computer can drive the densitometer to scan the ink measuring strips so long as the ink measuring strips are at known positions with respect to the test sheet and the test sheet is at a known position with respect to the X-Y positioning mechanism. One method of insuring that the test sheet is at a known position with respect to the X-Y positioning mechanism is to lay the test sheet against stops. Since these stops project above the surface upon which the test sheet is laid, however, the stops may interfere with the scanning of the optical densitometer.
To avoid the use of stops for alignment of the test sheets, the actual position of the test sheet upon a sheet support may be sensed. As described in West German Pat. No. 3,232,490, the position of the test sheet on the sheet support is detected photoelectrically by arrays of optical sensors. The scanning of the ink measuring strips on the test sheet is then adjusted based on the position of the test sheet on its support.